As is well known communications cables may be subjected to abuse such as, for example, attack by animals, mechanical abrasion and crushing. Attacks by gophers on buried cable and by squirrels on aerial cable have been a continuing concern. Gophers, for example, have been shown to exert biting pressures as high as 124,044k Pa. Cables which have an outside diameter below a size of about 1.07 cm in diameter and which do not have suitable, protective armor are more apt to be damaged than larger cables because the animals can bite directly down on them. For larger size cables, only a scraping or raking action takes place. In fact, on cables exceeding about 5.1 cm in diameter, gopher attack is rarely observed.
An excellent discussion of this problem was presented at the 25th International Wire and Cable Symposium. A written version of that presentation appears beginning at page 117 in the proceedings of that conference being authored by N. J. Cogelia, G. R. LaVoie, and J. G. Glahn and being entitled "Rodent Biting Pressure and Chewing Action and Their Effects on Wire and Cable Sheath".
Many rodents, and in particular the plains pocket gopher (Geomys bursarius) and the fox squirrel (Scirus niger), habitually chew on communication cables. Such chewing frequently causes immediate damage to the cable's wiring or optical fibers or results in damage when rain water or other moisture enters holes gnawed through protective outer jacketing. In the case of rodents, chewing on objects which are tough in composition is necessary to prevent their ever-growing incisor teeth from overgrowing. Thus, for relatively small cables, simply providing a relatively hard outer cable jacket, such as rigid polyvinyl chloride (PVC), for example, is insufficient and may even provide an inducement rather than a deterrent to chewing by rodents.
Because it is frequently undesirable or impractical simply to kill the rodents, it is necessary to devise non-lethal apparatus and methods whereby rodents will be prevented or deterred from attacking cables. Numerous behavioral studies have been performed which show that behavioral barriers can be induced by causing an undesirable effect to result immediately after an act such as chewing, for example, is performed by an animal. The animal then associates the undesirable result with the immediate preceding act and thereafter changes its behavior to prevent the undesirable result. In one prior art patent, this knowledge is utilized to incorporate in a cable assembly, a non-lethal but toxic chemical agent which is capable of causing rapid illness thereby inducing a behavioral barrier resulting in a behavioral change opposed to the cable chewing.
It has been found that an effective way to protect directly exposed cables from rodent attack is to wrap them in a metallic shield, preferably one which is longitudinally applied. The prior art is replete with patents relating to metallic sheath systems for copper core cables such as one comprising an aluminum shield enclosed by a carbon steel shield with each having a longitudinal seam. This sheath system offers protection from mechanical damage, and its cost is quite low because it is made in a single pass at relatively high line speeds. However, the use of a shield which is made of carbon steel occasionally has resulted in long term failures, even in cables larger than 2 cm. Failure may occur because the underlying steel shield may become exposed when rodents violate the jacket. Once exposed, the steel shield, which withstands the initial attack by rodents, may corrode readily. This may render it ineffective for general mechanical protection and for protection from any subsequent rodent attack. In this regard, it should be pointed out that gophers are territorial animals which repeatedly return to areas previously occupied by them. Therefore, it is not uncommon to experience secondary attacks in the same location along a cable. The prior art also includes the use of stainless steel shields to protect against rodent attacks. Stainless steel is used so that after an initial attack, where the steel layer has been exposed, the shield does not corrode readily and can withstand repeated rodent attacks over time. For larger diameter cables, the use of a corrugated shield having a longitudinally overlapped seam generally has provided sufficient protection. However, in the smaller sizes, wherein rodents have been able to encompass the cable with their teeth and pull open the seam, inadequate shield arrangements can lead to failures.
In another prior art approach to rodent protection, an all-dielectric optical fiber cable is disposed within an extruded plastic duct having an inner diameter which is significantly larger than the outer diameter of the cable. Such a structure is disclosed in an article entitled "Fiber Cable Wears An Extruded jacket" which was authored by G. J. Beveridge, et al., and which appeared in the Apr. 15, 1985 issue of Telephone Engineer and Management beginning at page 100. The outer diameter of the duct is sufficiently large to prevent rodents from enveloping the duct with their jaws. A cable installation having an additional duct adds to the cost of the installation.
Notwithstanding the hereinabove described solutions, others have been sought after. The use and handling of toxic chemicals is certainly not desired. Ordinary steel shields may corrode and stainless steel shields are not impervious to lightning. Also, the expense in the material cost and the application of an extra duct is a solution which certainly stimulates innovation.
The sought-after cable must be cost-effective and easily manufactured. Hopefully, it provides the cable with protection against rodent attacks without compromising other properties of the cable such as, for example, its flexibility. Further, the sought-after protection must be such that it can be included as a portion of the sheath system for use in any cable, be it one which includes optical fibers or metallic conductors.
There is also a desire that the sought-after cable include an all-dielectric sheath system. Cables which are installed in areas characterized as medium to high lightning areas may be damaged by lightning strikes. Metallic sheathed fiber optic cables can be affected by lightning in several ways. Thermal damage may be caused by the heating effects of the lightning arc whereas in buried cables, mechanical damage may result causing crushing and distortion of the sheath.
As may well be imagined, a cable having an all-dielectric sheath system which provides protection against rodent attacks as well as lightning would be welcomed. Such a cable would provide other advantages. An all dielectric cable avoids problems associated with corrosion. In addition, in the past, the oxidation of metallic sheaths including aluminum has led to the generation of hydrogen which could cause optical fiber transmission losses. An all-dielectric cable sheath system avoids this problem. Also, an all-dielectric cable sheath system offers survivability to electromagnetic pulse (EMP) effects and can be made immune to satellite detection.
An animal-resistant cable which is disclosed in application Ser. No. 194,760 which was filed on May 17, 1988 in the names of C. J. Arroyo and P. D. Patel comprises a core which includes at least one transmission media, and a sheath system which encloses said core. The sheath system is non-metallic and includes a plurality of longitudinally extending preformed strength members. The longitudinally extending members, which are referred to as segments, form a shell that encloses the core. In order to provide sufficient flexibility for the cable to be plowed into the ground in a buried installation, the number of shell segments must be such that each encloses less than one half of the periphery of the core and the shell segments are stranded helically about the core. An outer jacket comprising a plastic material encloses the shell segments.
Although the above-described animal-resistant cable represents a significant advance over the prior art, tests have shown that smaller cables which include such a shell structure still are somewhat vulnerable to attack by rodents. What is desired is a cable structure which is substantially animal resistant over a wide range of outer cable diameters. Further, there has been a desire to reduce the diameter of the cable.
Seemingly, the prior art does not offer a cable structure which is free of the above described shortcomings of presently available cables and which is effective over a wide range of outer cable diameters. There are cables having all-dielectric sheath systems and there are cables which provide protection against rodent attacks and lightning, but there appears to be no sheath system which provides both kinds of protection over the desired wide range of sizes. Such a cable which has long been sought-after must be cost-competitive. Further, it must include a sheath system which is easily removed to access safely the core which could include, for example, optical fibers.